The present invention relates to a method, a communication network and a control unit for cyclically transmitting data, particularly in the form of Ethernet telegrams.
The Ethernet is the most widespread technology by means of which data may currently be transmitted at a rate of up to 100 million bits per second within local communication networks, so-called Local Area Networks. LANs are local communication networks limited to a geographical area and comprise at least a control unit and several work stations, also called nodes, as participants, which are connected by means of a transmission path such as a coaxial cable, a glass fibre cable or a twisted pair cable.
LANs are operated by means of a network operating system and a unified network protocol. The Ethernet represents a potential network protocol and thereby supports varying kinds of communication protocols, such as the TCP/IP protocol or the IPX protocol. In the OSI layer model, the international reference model for data communication in networks consisting of a layer stack comprising seven layers, an amount of protocols being defined for each layer and the protocols providing their services to the respective next higher layer, the Ethernet protocol is assigned to the second layer, the so-called conductor layer. In this conductor layer, the data to be sent are concentrated to form telegrams to which specific information relating to the respective communication protocol are added. In the network, the conductor layer is responsible for transporting the data telegrams from one participant to the other and for detecting errors.
In the Ethernet concept, the conductor layer is divided up into two levels, the first level adding to the data a header, a so-called start identification, which comprises information required by the receiving protocol for a correct data transmission. On the second level of the Ethernet protocol, the telegram is then encapsulated by means of an additional preamble and an end section, a so-called check sum, for transporting it from participant to participant. By means of such an Ethernet telegram, data having a length of up to 1500 bytes may be transmitted, whereby fixed break intervals have to be observed between the individual Ethernet telegrams.
For sending and receiving the Ethernet telegrams via the Ethernet transmission path, an Ethernet controller is typically responsible, also referred to as media access controller, which is connected between the control unit and the Ethernet transmission path. The MAC controller generally comprises a sending and a receiving shift register in order to decouple the Ethernet transmission path from the physical memory of the control unit. Current Ethernet controllers further typically comprise a possibility of directly accessing the physical memory, a so-called direct memory access mode, by means of which the Ethernet telegrams to be sent and to be received may be directly stored in the memory or retrieved therefrom, respectively, in a time-saving manner.
Ethernet protocols are predominantly used in office communication networks. Due to the advantages of the Ethernet concept when using standard hardware and software components as well as the possibility of achieving high data transmission rates with a simple network technology, Ethernet communication networks are also increasingly used in industrial manufacturing for the data exchange between work stations. When using the Ethernet protocol in automation, the real time capability of the Ethernet data transmission has to be ensured. When controlling machines it is typically required to carry out a cyclic processing of a control task essentially without time variations, i.e. so-called jittering, the standard requirement being responded within a predictable reply time.
If Ethernet telegrams are to be cyclically transmitted e.g. within the framework of a real time application running on the Ethernet network in order to address sensors and actuators connected by the Ethernet transmission path, the control unit in each control cycle transfers the respective Ethernet telegrams to its MAC controller for transmission. Thereby, prior to the transfer to the MAC controller, the control unit automatically adds the break intervals, start identifications, preambles and check sums defined in the Ethernet transmission standard (IEEE 802.3) to the data to be sent. The MAC controller then uploads the Ethernet telegrams into its transmission shift register, particularly by means of the DMA mode, and if a particular charging level of the transmission shift register is achieved, the MAC controller starts to transmit the Ethernet telegrams on the Ethernet transmission path.
Present-day manufacturing and processing techniques are increasingly marked by rapid control tasks. As a result, the control unit in the communication network used for industrial automation has to react to standard requirements with increasing speed. In this manner, response times in the microsecond range are required for controlling technical processes, e.g. in the plastics industry. In order to safeguard such rapid reaction times, a high performance processing unit is necessary for the control unit in LAN networks in order to guarantee cycle times in the microsecond range which in turn results in high hardware costs.
There is a need for a method for transmitting data, a communication network and a control unit for such a communication network by means of which data may be cyclically distributed among data communication participants at high frequencies.